Due to superior vibration damping and impact absorption properties as well as excellent dynamic characteristics under high loads, durability and permanent deformation resistance, polyurethane elastomers having a microcellular structure have been extensively used in helper springs for bicycles, motorcycles, automobile suspension devices, furnitures or high-vibrating matrixes. In particular, as polyurethane elastomers prepared by using naphthalene diisocyanate (NDI) have superior bending fatigue resistance, they are widely applied in cases where high durability is required, such as in helper springs for bicycles, motorcycles, automobile suspension devices, furnitures or high-vibrating matrixes.
Typical polyurethane elastomers are prepared by reacting a polyol with a diisocyanate-based material. Conventional methods of preparing polyurethane elastomers with using NDI are performed by, for example, reacting NDI alone, or in combination with 4,4-diphenylmethane diisocyanate (p-MDI), with a polyol to prepare a polyurethane prepolymer containing a terminal isocyanate (NCO) group, followed by mixing the prepared polyurethane prepolymer containing a terminal NCO group with water, a catalyst, a surfactant, an antioxidant, a chain extender, etc. to obtain a prepolymer blend, and then foaming the prepolymer blend with a foaming machine to prepare a polyurethane elastomer.
In addition, as disclosed in U.S. Pat. No. 4,334,033, a method of preparing a polyurethane elastomer includes 1) reacting toluene diisocyanate (TDI) with polyethylene glycol to form a hydroxyl-terminal prepolymer, followed by reacting the prepolymer with NDI to prepare a polyurethane prepolymer having a terminal NCO group; and 2) then reacting the hydroxyl-terminal prepolymer and the polyurethane prepolymer having a terminal NCO group with butanediol to obtain a polyurethane elastomer.
However, in the case where NDI alone is reacted with a polyol, since the prepolymer thus prepared has a short shelf life and high viscosity, it requires operation at a high temperature of at least 100° C., thereby undesirably making it difficult to handle feeds. Also, when p-MDI or TDI is additionally used to solve this problem, the properties of a polyurethane elastomer is deteriorated due to the structural feature of p-MDI or TDI. Therefore, there is a need to develop polyurethane elastomers having improved viscosity and superior properties.